My recollection with reddit was that it was a 'tipping point' sort of process, and it went fairly quickly from being good to full of politics articles with roving bands of morons voting down everything they disagreed with.
The other thing is: if you want to discuss the election, or Iran, or Iraq, or unions, or the economy, you can do that on just about every forum on the internet. But if you want to discuss, say, VC-backed vs bootstrapped businesses, or how much stock an eighth employee should get, or [I dunno, insert third example here], there are very few places where you'll find a critical mass of people who know what they're talking about.
I was surprised to read this because your comments have usually seemed very good to me. But then I ran across http://news.ycombinator.com/item?id=242733 - so I'll concede the point on that thread. :)